The invention relates to a metallic heat exchanger tube.
Metallic heat exchanger tubes of this type are used, in particular, for the condensation of liquids from pure substances or mixtures on the tube outside. Condensation occurs in many sectors of refrigeration and air conditioning technology and also in process and energy engineering. Tube bundle heat exchangers are often used, in which vapors from pure substances or mixtures are liquefied on the tube outside and at the same time heat a brine or water on the tube inside. Such appliances are designated as tube bundle condensers or tube bundle liquefiers.
The heat exchanger tubes for tube bundle heat exchangers usually possess at least one structured region and also smooth end pieces and, if appropriate, smooth intermediate pieces. The smooth end or intermediate pieces delimit the structured regions. So that the tube can be installed in the tube bundle heat exchanger without difficulty, the outside diameter of the structured regions should be no larger than the outside diameter of the smooth end and intermediate pieces. The high-performance tubes customary nowadays are somewhat more efficient than smooth tubes of the same diameter by about the factor four.
Various measures are known for increasing the heat transfer during the condensation on the tube outside. Ribs are often attached on the outer surface of the tube. As a result, primarily, the surface of the tube is enlarged, and consequently condensation is intensified. For the heat transmission, it is especially advantageous if the ribs are formed from the wall material of the smooth tube, since there is then optimal contact between the rib and tube wall. Ribbed tubes in which the ribs have been formed from the wall material of a smooth tube by means of a forming process are designated as integrally rolled rib tubes.
It is prior art to enlarge the surface of the tube further by the introduction of notches into the rib tips. Furthermore, due to the notches, additional structures arise which positively influence the condensation process. Examples of notches for rib tips are known from the publications U.S. Pat. Nos. 3,326, 283 and 4,660,630.
Commercially obtainable ribbed tubes for liquefiers nowadays possess on the tube outside a ribbed structure with a rib density of 30 to 45 ribs per inch. This corresponds to a rib division of approximately 0.85 to 0.56 mm. Ribbed structures of this type may be gathered, for example, from the publications DE 44 04 357 C2, US 2008/0196776 A1, US 2007/0131396 A1 and CN 101004337 A. Limits are placed on the further rise in performance as a result of an increase in the rib density by the inundation effect which occurs in tube bundle heat exchangers: with a decrease in spacing of the ribs, the interspace between the ribs is flooded with condensate due to the capillary effect, and the flow-off of the condensate is impeded due to the fact that the channels between the ribs become smaller.
Furthermore, it is known that increases in performance can be achieved in liquefier tubes in that, with the rib density remaining the same, additional structural elements are introduced between the ribs in the region of the rib flanks. Such structures may be formed by gearwheel-like disks on the rib flanks. The material projections which in this case occur project into the interspace between adjacent ribs. Embodiments of such structures are found in the publications US 2008/0196876 A1, US 2007/0131396 A1 and CN 101004337 A. These publications show the material projections as structural elements with planar boundary faces. The planar boundary faces are a disadvantage, since the condensate formed does not experience, on a planar face, a force which is induced by the surface tension and which would remove it from the boundary face. An undesirable liquid film is therefore formed, which may persistently obstruct the transmission of heat.